X-ray genenrator tubes



Dec. 2, 1958 A; SCHRAM 2, 3

X-RAY GENERATOR TUBES Filed March 25, 1957 Fig.2

FOCAL TEMPERATURE t TEMPERATURE OF TIIE FOCAL Rm TEMPERATURE F THE ANODEDISC .flfi'MME Fig.7 1

Inventor By wit -e, WM

2,853,083 Patented Dec, 2,.

X RAY GENERATOR TUBES Antoine Schram, Montgeron, France, assignor toCompagnie Generals de Radiologie, Paris, France, a cor- This inventionrelates to X-ray generator tubes. In tubes such asX-ray generator tubes,the dissipation of heat from the anode is the principal problemencountered, because almost the whole of the energy of the electron beamis there transformed into heat. In particular, it is the speed ofcooling of the anode which limits the rate of making successiveexposures, or exposures following viewing. For X-ray generator tubeswith a rotatable anode, for example the cooling takes place principallyby radiation from the ano-de.

It. is moreover necessary that the anode, at least the part bombarded bythe electrons, shall be constituted up to a certain depth (at least 10microns) of a metal having a high atomic number and which is veryrefractory. Hitherto generally it is tungsten that is used. Particularlyin thetcaseof a rotatable anode, it is desirable that the thermal.capacity shall be great, in order :to allow the storage of a largequantity of heat before reaching the maximum admissible temperature. a

It is therefore an object of thepresent invention to provide an improvedconstruction of anode for X-ray generator tubes in which the speed ofcooling of the anode is considerably increased.

It is a further object of the invention to provide such an. anode havingan increased thermal capacity.

It .is a specific object oftheinvention to provide an anode for an X-raygenerator tube in which at least the part of the anode surface which isbombarded by electrons, consists of the metal rhenium.

It is yet another object of the invention to provide improved X-raygenerator tubes incorporating such anodes.

Other objects and advantages of the invention will appear from thefollowing description taken in conjunction with the accompanyingdrawing, in which:

Figure 1 is a. diagrammatic elevational view. of. an X- ray generatortube havinga rotatable anode, which may be formed according to thepresent invention,

Figure 2 is a plan view or" the anode assembly of the tube in Figure 1,r

Figures 3 and 4-are explanatory graphs,

Figure 5 is a cross section, on a larger scale, through an anodeassembly according to the present invention, in tended for the tubeshown in Figures 1 and 2, and

Figures 6 and 7 are fragmentary cross-sections of modified constructionsof anode assemblies according to this invention.

Referring to Figures 1 and 2, there is shown. by way. of example, a typeof X-ray generator tube having a rotatable anode. The glass envelope ofthis tube contains, in a very high vacuum, the cathode system 2, ofwhich the filament or filaments 3 in the concentrating member arelocated opposite to the conical portion of a disc 7 forming the anode ofthe tube, which is joinedby a rod of molybdenum 6 and a screw.8, to therotor 5. By applying a high tension voltage so that the cathode isnegative and the anode is positive, .a beam of electrons emitted fromthe incandescent filament, bombard the focal. seat United States Patent"Ofiice 10 andproduce X-radiation at this point. Thetenergyistransformed almost entirely into heat which, leaving the focal seat 10,rapidly reaches the focal ring 9 and finally the whole mass of the anode7.

The tube structure above described is of a known type and the rate atwhich successive exposures maybe made, or at which exposures may be madefollowing a viewing,

depends upon the thermal capacity of, and the speedof cooling from theanodedisc 7.

As shown in the full line curve in the graph of Figure. 3, during aradiographic exposure, the. focal temperature starts from T risessuddenly to T then during itheexposure which lasts from the instant t torises from T to T This increase follows that of the focal areaor ring 9which, during the same time, passes from T to T ,..as shown in thebroken line curve. Finally, as showninthe chain line'curve, thetemperature of the anode disc7 also rises during the exposure andpasses. from T to T "T being lower than T The ageing of the anode 7causes a fall of X-radiation depending on the temperature of the focalseat 10. .It is thus plain that it is of interest to start from atemperadissipation which follows the .StephamBoltzmann law:

, Where:

W=the heat radiatedin watts E=the total emissivity of thesurface at thetemperature T 6='constant=5.67.1O- W/cn1. .l

T=the temperature of the surface in K. 8 T =the ambient temperature inK.

S=the radiating surface in cm. (apparent surface) For the temperaturesin question it is possible to neglect T compared with T. For given'TandS,.it is then necessary to increase 6 if it is desired to improve W.Now,

for tungsten e l1as,:-more. or less, the following values:

At 1000 K 0.114 At 1500 K 0.192 At 1700 K 0.222 At 2000 K *0.260 At-2500K '0;s03

The theoretical maximum e=.1 which? corresponds: to a black body whichshows that .from the pointlofvview of 1 thermal dissipation .atugnstensurfaceis tHOt ideal.

Methods are .known. for increasing .tthe actual surface area of.ananode, such as sand blasting, .chemicalattack or electrochemicalaction, which-:allowsetobeincreased totacertain extent.

A certain-numberjof .otheryprocesseshavef also "been proposed whichconsist 'in. a deposit. of 1 anothertmaterial, increasing the actualsurface. andat the same tirne the intrinsic emissivity. ,These,processes 'llJOWfiVGI? present serious disadvantages for atubefiofwwhich .the anode operates at high temperature such as anX-raywgenerator tube, which is easilyunderstood-because-thetmajorityofthese processes are proposed forelectron tubes of which tthe anode ICITlEtlIlS fiifi'lOW temperature. 'lhusyit hasbeen suggested to deposit refractory carbides directly byelectrophoresis, or by depositing a meta'lliccxide subseit anode of anX-ray generator tube, the adherence of the deposit is not alwayssatisfactory, and it is necessary to spare the focal area of the anodeso as not to reduce the etficiency of the X-rays and also because mostof these deposits do not support very high temperatures. These processesare therefore neither reliable nor economical; it is very diflicult toobtain deposits which are regular and only wheredesired, and without theliberation of a excessive quantity of gas at high temperature. There areclassical processes for the 'carbonisation of nickel anodes, giving avery efficient cooling. But these anodes cannot operate at thetemperatures usually met Within tubes such as X-ray tubes.

The present invention eliminates these disadvantages by employing themetal rhenium inorder to cover the whole surface, or apart of thesurface, of the anode. In effect, the thermalemissivity of rhenium ishigher than that of tungsten at all temperatures encountered during theoperation of the tube. By reason of its atomic number, which is higherthan that of tungsten (.75 instead of 74) and by reason of its meltingpoint being near to that of tungsten and on'account of its very lowvapour pressure, the focal surface can also be covered with rhenium.

The present invention also provides X-ray generator tubes with an anodehaving an increased thermal capacity. In effect, the use of rheniumallows a refractory base to be used with highe thermal capacity thantungsten since one is not confinedby the necessity of having a highatomic number. The refractory base may be, for example-, molybdenum,graphite or boron. It is sulfi- 'cient to have on the focal area wherethe X-rays are produced, a thickness .of rhenium sufficient so that allthe X-radiation originates from the rhenium. Molybdenum in particular isinteresting.

FigureS shows diagrammatically, asection through 'rhenium having athickness of at least microns on the focal ring. The remainder of thesurface of the molybdenum can also be covered with rhenium 14 of thesame thickness or of less thickness. According to another embodiment ofthe invention, the non-focal surface 14 is blackened by a known process.

Figure 6 shows a diagrammatic fragmentary section of part of anotheranode disc, in which the refractory base structure 11, for example ofboron, is covered directly with a layer of rhenium 13 extending over thelower surface of the anode.

Figure 7 shows a diagrammatic fragmentary section of part of anotheranode structure according tothis invention, in which the anode consistssimply of a massive disc 15 of rhenium. 7

By the present invention, it is therefore possible to obtain anodestructures having a higher thermal capacity than tungsten anodes, whilstincreasing at the same time the amount of X-radiation.

The metal rhenium presents, moreover, physical and mechanicalcharacteristics which are advantageous at high temperatures, and whichare used by the present invention. For example, a rotating anode oftungsten and rhenium or of rhenium alone allows operation at a highertemperature than an anode of tungsten.

Figure 4 shows the speed of cooling of a tungsten anode in curve A andof an anode according to the present invention in curve B. a

It is possible to choose between several methods of obtaining a layer ofrhenium on a refractory base. Preferably an electrolytic method is used.A bath having a base of perrhenate of potassium is very practical:

KReo, V 11 g./l. n so, (d=1.84) pH 0.9

Temperature 20 to 75 C. Current density 5 to 15 A/dm. Anode Platinum Itis of advantage firstly to deposit a very thin layer of the order of onemicron, followed by a flash in hydrogen to 1000 C. Then it is possibleto deposit the desired thickness by proceeding with intermediate flashesin hydrogen. The electrolytic process is of very great advantage if itis desired to obtain a layer on one part only of the surface of therefractory base and if it is desired to vary the thickness of the layerfrom one place to another.

Another recommended process is the deposition in vapour phase, by thedecomposition of a halogenide of rhenium, in particular ReCl on a baseheated to between 500 and 1500 C. in vacuum, or in an inert gas.

Metallisation by spraying and calcining also enables a layer of rheniumto be obtained on a refractory base.

Whilst particular embodiments have been described, it will be understoodthat various modifications may be made without departing from the scopeof this invention. Thus, although particular reference has been made toX-ray generator tubes having a rotatable anode, the invention mayequally be employed in such tubes having a non-rotatable anode.

I claim:

1. An anode for an X-ray generator tube, in which at least the part ofthe anode surface which is adapted to be bombarded with electronsconsists of the metal rhe mum.

2. An anode for an X-ray generator tube, consisting of a massive disc ofthe metal rhenium.

3. An anode for an X-ray generator tube, consisting of a core of arefractory material covered with a layer of rhenium over at least a partof its surface and extending over at least the focal area of the anode.

4. An anode as claimed in claim 3, in which the core of refractorymaterial consists of molybdenum.

5. An anode as claimed in claim 3, in which a layer of tungsten isprovided on a part. of the surface of the core, under the layer ofrhenium.

6. An anode for an X-ray generator tube consisting of a core of arefractory material, a layer of tungsten covering at least a part of thesurface of the refractory material and a layer of rhenium over saidlayer of tung; sten and covering at least the focal area of the anode,to a depth of at least ten microns.

7. An anode as claimed in claim 6, in which the base of refractorymaterial consists of a metal other than tungsten or rhenium. 5

8. An X-ray generator tube comprising an envelope containing a cathodeassembly, a filament associated with the cathode assembly and an anodespaced from said cathode assembly in which at least the focal area ofthe anode which is bombarded by electrons to produce the X-radiationconsists of the metal rhenium.

9. An X-ray generator tube comprising an envelope containing a cathodeassembly, a filament associated with the cathode assembly and an anodespaced from said cathode assembly and made of the metal rhenium.

10. An X-ray generator tube comprising an envelope containing a cathodeassembly, a filament associated with the cathode assembly and an anodespaced from said cathode assembly, said anode comprising a core ofrefractory material, and a layer of rhenium'extending over at least thefocalarea of the core which is bombarded by electrons to produce theX-radiation. i

11. A tube as claimed in claim 10, in which the core of refractorymaterial consists of molybdenum.

12. A tube as claimed in claim 10, in which the layer of rhenium is atleast ten microns thick over the focal area of the anode.

. 13. An X-ray generator tube comprising an envelope containing acathode assembly, a filament associated with the cathode assembly and ananode spaced from said cathode assembly, said anode consisting of a coreof refractory material and a layer of rhenium extending only over thefocal area of the core which is bombarded by electrons to produce theX-radiation, said layer of rhenium having a thickness of at least tenmicrons.

14. A tube as claimed in claim 13, in which the core is made ofmolybdenum.

15. An X-ray generator tube comprising an envelope containing a cathodeassembly, a filament associated with the cathode assembly and an anodespaced from said cathode assembly, said anode consisting of a core of arefractory material, a layer of tungsten covering at least the focalarea of the refractory material and a layer of rhenium extending oversaid layer of tungsten and having a thickness of at least ten microns.

16. An X-ray generator tube comprising an envelope containing a cathodeassembly, a filament associated with the cathode assembly and an anodespaced from said cathode assembly, in which at least the focal area ofthe anode which is bombarded by electrons to produce the X- radiationconsists of the metal rhenium, and the remainder of the surface of theanode is blackened.

References Cited in the file of this patent UNITED STATES PATENTS2,482,053 Zunick Sept. 13, 1949 2,490,246 Zunick Dec. 6, 1949 2,762,725Saunders Sept. 11, 1956 2,762,726 Saunders Sept. 11, 1956

6. AN ANODE FOR AN X-RAY GENERATOR TUBE CONSISTING OF A CORE OF AREFRACTORY MATERIAL, A LAYER OF TUNGSTEN COVERING AT LEAST A PART OF THESURFACE OF THE REFRACTORY MATERIAL AND A LAYER OF RHENIUM OVER SAIDLAYER OF TUNGSTEN AND COVERING AT LEAST THE FOCAL AREA OF THE ANODE, TOA DEPTH OF AT LEAST TEN MICRONS.